In right-hand traffic countries the left turn is perhaps the most dangerous of driving maneuvers, simply due to its nature, i.e. you are turning, often from a standstill, across another lane of traffic that's moving in the opposite direction.
Vehicles turning left into oncoming traffic result in many injuries and even fatalities at intersections. When you turn left, you are crossing one or more lanes of oncoming traffic, presenting your vehicle's broadside to any approaching vehicles. Thus, especially the passenger's side front seat is put into harm's way. If your turning vehicle gets struck, any occupant of the passenger's side front seat takes the most severe impact. Due to the geometry of passenger cars, the protection afforded by the vehicle body against a side impact is usually substantially less than that provided against a front- or rear-end impact.
Also, it is possible that another vehicle will attempt to overtake you while you are preparing to turn. Thus, your vehicle may get struck by the overtaking vehicle should you proceed with the turning maneuver.
Furthermore, when waiting to make a left turn drivers sometime tend to turn their wheels slightly left. As a result thereof the vehicle could be pushed into oncoming traffic if accidentally hit from behind by another vehicle.
In left-hand traffic countries the above applies conversely for right turns.